Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. The RF wave may encode one or more commands that instruct the tags to perform one or more actions.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded information can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Accordingly, when a reader reads a tag code, information can be learned about the associated item that hosts the tag, and/or about the tag itself.
An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In some RFID tags the power management section includes an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
RFID readers often communicate with populations of tags, performing operations such as inventorying or otherwise interacting with the tags. To perform such communications, the reader must establish a reliable communications link with the tags. Unfortunately, the real-world environment where readers and tags operate contains unavoidable RF noise. White noise (noise with a flat frequency spectrum) is always present; colored noise (noise that is more prominent on some frequencies than on others) may be present as well. Some of this noise will fall within the channel or channels used by a reader and tags to communicate, degrading the communications.
As one example, thermal fluctuations in the RF environment cause broadband white noise at RF frequencies. As another example, fluorescent lights can amplitude modulate (AM) the RF waves used in RFID communication, creating sidebands (signal components) at frequencies offset from the frequencies of the RF waves. Such sidebands are one example of colored noise. In some cases these AM sidebands fall within the tag reply spectrum, and can cause symbol decoding errors in the reader. Traditional RFID readers employ channel filters, as well as data filters matched to the tag signal waveforms, to attenuate noise that falls outside the tag reply spectrum. These filters are effective against white and colored noise that falls outside the reply spectrum, but they cannot mitigate noise, such as the above-mentioned colored noise from fluorescent lights, that falls within the tag reply spectrum. Furthermore, these filters do not reduce inter-symbol-interference (ISI) that can be introduced by the RF channel or by the reader analog front-end electronics.